On the nature of bonding in a new boronyl species Zn2(BO)2: a linear four-center two-electron σ bond.

ABSTRACT

The Zn-Zn bond as one of the metal-to-metal bonds in clusters and molecules is of fundamental interest in many areas of natural science. Neutral boronyl can be viewed as a σ radical and is found in boronyl metal complexes. However, a complex with the Zn-Zn bond stabilized by boronyl ligands has not been found so far. Herein, we report on the computational design of the simplest case of such a system linear D∞h OBZnZnBO. The structural and electronic properties and chemical bonding on a series of zinc complexes Znx(BO)y (x = 1,2; y = 1,2) with boronyl as ligands have been studied using quantum chemical calculations at the B3LYP and PBE0 levels, respectively. For the Zn2(BO)2 cluster, the linear D∞h OBZnZnBO is the global minimum, in which the calculated Zn-Zn bond length of rZn-Zn = 2.400 Å at the B3LYP level, which appears to be close to the latest recommended covalent radii (2.40 Å) of the proposed single bond covalent radii of the Zn-Zn bond. Chemical bonding analyses show that D∞h OBZnZnBO possesses a linear four-center two-electron (4c-2e) σ bond. The σ bond framework has a contribution of Zn orbitals 54% and B orbitals 44%, which involve Zn 4s 20% and 4p 34%, and B 2s 28% and 2p 16%, respectively. Furthermore, the D∞h HZnZnH and NCZnZnCN clusters also exhibit one linear 4c-2e σ bond due to the secondary contribution from the H s and C sp components, respectively. The linear 4c-2e σ bond greatly stabilizes the dizinc complexes. D∞h OBZnZnBO is thermochemically stable with respect to the possible formation channel at room temperature, whereas the formation energy of the exergonic channel, 2ZnBO (C∞v, 2Σg) → OBZnZnBO (D∞h, 1Σg), is evaluated to be -58.75 kcal mol-1 at the B3LYP level. Thus, D∞h OBZnZnBO as the first observation of the Zn-Zn covalent bond in zinc complexes with boronyl as ligands may be synthesized in laboratories in the near future.